Shoot a few frames against different colored backgrounds to see if you notice anything in the images. If no, than you are fine. If there is noticable change, than contact Canon or a authorized dealer to have it cleaned and maintained. Been 10 years so it is due for a check-up maybe. Good news is the EF 300 f/4 L IS is a stupendous lens so enjoy. Many a newb do not get to start off so lucky. :mrgreen:

Guys regardless if it's visible or not, if it's fungal growth get it cleaned ASAP. Fungus only spreads, and gets worst, and if left too long will etch the glass. Not something you want to have happen on a $2000 lens.